How much it cost is dependent on where you live. If you live in an area where you pay taxes that support an ambulance service, then it is free. They will still charge your insurance, but if you do not have any, no bill. If you live in an area where some services are covered by local taxes, you will get a bill, but it will be at a reduced rate.
If you live in an area where the ambulance service receives no tax money, you will be responsible for the whole bill. If you have insurance, it will cover a portion of the bill.
Now, irrespective of what area you live in, the cost to operate are similar. An ambulance stocked with all the equipment cost about $300,000 on average. You have trained providers on there who expect to be paid. Most departments have stations that need to be purchased or rented and maintained. When you call 911 for help, someone has to be there to answer the phone and to dispatch the appropriate units. Someone has to work in the office to do billing. Someone has to run operations, training, scheduling, etc. All these people will get paid.